Since Crusader Kings II sees players controlling not a country but a ruling dynasty, sex, just like in real life, couldn't be more important. Failure to raise a suitable heir could face you with a Game Over screen, while especially virile kings might find a romantic affair shattering the reputation of their entire nation. Which is to say nothing of bastard children! Having leaked from your loins they have a stronger claim to your throne than most, which would make them a worryingly influential enemy. Oh dear!